A car alarm is an electronic device installed in a vehicle in an attempt to discourage theft of the vehicle itself, its contents, or both. Car alarms work by emitting high-volume sound (usually a siren, klaxon, pre-recorded verbal warning, the vehicle's own horn, or a combination thereof) when the conditions necessary for triggering are met, as well as by flashing some of the vehicle's lights, and (optionally) notifying the car's owner via a paging system and interrupting various electrical circuits necessary for the car to start.

Car alarms should not be confused with immobilizers; although the purpose of both may be to deter car theft, they operate in a dissimilar fashion. An immobilizer generally will not offer any audible or visual theft deterrence, nor require any additional input from the driver than from the driver of a non-immobilizer car.